This invention relates to a method and apparatus for inspecting a quality of cigarette paper of eventual cigarette units making use of air leakage therethrough.
In the manufacture of cigarettes by a cigarette manufacturing machine, defects such as faulty glueing at the time of glueing chip paper which connects a cigarette portion and filter portion and faulty winding at the time of winding of the chip paper which is liable to result in oblique glueing and one-end glueing in addition to the breakage of cigarette paper and pinholes therein can occur. The oblique glueing is a form of faultiness that both edges of the chip paper are glued together not in the circumferential direction but in an oblique direction. The one-end glueing is a faultiness in which one end of the chip paper is not glued in the circumferential direction but remains separated. In either case, a gap is produced between the chip paper and filter, so that an abnormal defective of appearance results.
It is a common characteristics of these faulty products that air leakage results in a defective portion or defective glueing portion when air is forced through the product.
Accordingly, the quality of the product is inspected by supplying air through the product. For example, where the product is a double cigarette, which consists of two eventual cigarettes and a filter sandwiched therebetween with a filter chip paper glued to these components, air is supplied through the double cigarette from one end or both ends thereof, and the air pressure is converted into an electric signal. This air leakage inspection makes use of a phenomenon that the air pressure is reduced by air leakage. The voltage level of the electric signal noted above is compared with a predetermined reference voltage level corresponding to a reference level of air leakage. The inspected double cigarette is inspected to be non-defective or defective depending on the result of the comparison.
Recently, various brands of cigarettes are produced, and it is in practice to use cigarette paper of different air permeabilities suited to the characteristics of various brands in order to provide different characters of quality and different fragrance or aroma features. Generally, the air permeability of cigarette paper varies from not more than 10 ml/min.multidot.cm.sup.2 of ordinary cigarette paper used for ordinary brands to 24 to 160 ml/min.multidot.cm.sup.2 of high air permeability kinds of cigarette paper with high density of stoma-like holes used for mild taste of aroma brands. In the case of the high porosity cigarette paper especially, variability of the air permeability occurs in the manufacture in a predetermined range with respect to a nominal air permeability. More specifically, with the same high porosity, the physical properties of cigarette paper, i.e., relative air permeability thereof, varies.
With the conventional apparatus, the detected pressure signal level of low air permeability cigarette paper is high compared to that of high air permeability cigarette paper. This means that even if the detected pressure signal level is reduced to a certain extent due to the detection of a defective product, with the variability of the air permeability thereof being beyond the permissible range, the detected signal level of voltage may be still higher than the reference voltage level. In this case, the double cigarette is deemed to be non-defective (the error of this case being referred to as first kind of error). Conversely, the detected pressure signal level of high air permeability cigarette paper is relatively lower than that of low air permeability cigarette paper. Therefore, it is liable that a sound double cigarette provides a detected pressure signal of lower level than the reference voltage level so that it is deemed to be defective (the error of this case being referred to as second kind of error). The errors as noted will reduce the precision of inspection making it difficult to obtain accurate decision as to whether a double cigarette is non-defective or defective and reducing the quality of the product.
When there occurs a phenomenon of continuously passing defective products or continuously excluding non-defective products in case where high porositity cigarette paper as noted above is used, it has been necessary to adjust a reducing valve while watching a manometer indicating the master pressure of the detected pressure with increasing amount of labour of workers.
In another aspect, a detector used for detecting the pressure usually consists of a pressure-to-electricity conversion element of semiconductor, the detector brings about a thermal drift due to changes in the room temperature caused by heat generation from the machine or other causes. The precision of inspection is reduced by the thermal drift.
In order to solve the problems arising from the variability of the air permeability noted above, it has been proposed to process the detected pressure signal with processing means consisting of averaging means (AGC means) and comparing means as disclosed in Japanese Patent Publication No. 56-46820 and Japanese Patent Preliminary Publication No. 51-82798. Particularly, the early published specification No. 51-82798 discloses an apparatus, in which the detected pressure signal is compensated for by a difference signal between a reference signal obtained in an open to-atmosphere state and detected pressure signal, thereby solving the problems of the thermal drift as well. The apparatuses disclosed in the disclosures noted above, however, basically effects decision only from the standpoint of the mean value, so that the precision of inspection in the population mean estimation is statistically low.